1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer and the like having a cartridge, and more particularly it relates to a color image forming apparatus.
2. Related Background Art
Recently, in many applications for image forming apparatuses such as an electrophotographic color copying machine, color printer and the like, a plurality (for example four--yellow color (Y), magenta color (M), cyan color (C) and black color (BK)) of developing devices have been disposed around an electrophotographic photosensitive drum. During the formation of a color image, these developing devices are sequentially driven as the photosensitive drum is rotated, so that an electrostatic latent image on the photosensitive drum is developed with each color toner. Thereafter, the developed image is transferred onto a sheet.
By the way, it has been requested that such color image forming apparatus should also be made small-sized and have easy maintenance. To meet such requirement, the technique in which the process elements such as a photosensitive drum, developing device and the like are contained in a cartridge, which was usually used with a monochromatic-color image forming system has been adapted to the color image forming apparatus, thereby permitting easy maintenance by changing or replacing the cartridges by an operator.
FIG. 3 is an elevational sectional view of a conventional color image forming apparatus. As shown, a cartridge C is removably mounted within an image forming apparatus 110. The cartridge C has a housing 104 including a photosensitive drum 106, a cleaner having a cleaning blade 117, color developing devices 105Y, 105M, 105C, 105BK and a corona discharger 100 therein. In addition, the image forming apparatus 110 is provided with an optical system 114' for scanning with a laser beam the photosensitive drum 106 which is uniformly charged by the corona discharger 100, a sheet supply cassette 101 for supplying a plain paper as a transfer sheet, a rotatable drum 102 for holding the sheet and adapted to be rotated by several revolutions to transfer the developed images formed on the photosensitive drum 106 onto the sheet, and a fixing device 103 for thermally fixing the non-fixed toner image transferred to the sheet.
By the way, the color image forming apparatus differs from the monochromatic-color image forming system in the point that, in the color image forming apparatus, since the images are developed by a plurality of developing devices, it is necessary to prevent the mixing of colors and to develop the images with each color toner by sequentially driving one of the developing devices. Thus, in the color image forming apparatus, it is required to provide means for changing the drive of the developing devices so that, during the development with a certain color toner, only the developing device associated with such color toner is driven, but the other developing devices are not driven.
As mentioned, when the cartridge is adapted to the color image forming apparatus, although the above-mentioned drive changing means is required, the applicant has proposed a technique wherein a drive changing mechanism is arranged at the side of the image forming apparatus and the drive changing mechanism is connected to drive shafts of the developing devices when the cartridge is inserted into the apparatus, as described in the Japanese Patent Application No. 2-90395 (filed on Apr. 6, 1990).
However, in this proposed technique, since there are a plurality of connection portions between the drive shafts of the developing devices at the cartridge side and the drive changing mechanism at the apparatus side, when the cartridge is mounted within the apparatus, the positioning of the connection portions will be complicated, with the result that it is very hard to correctly position the connection portions only by the insertion of the cartridge into the apparatus.
On the other hand, the conventional cartridge is generally provided with a driving force transmitting means such as gears, belts and the like for transmitting a driving force to the photosensitive drum and the developing devices disposed within the cartridge. It is also true regarding the monochromatic-color cartridge having only one developing device. Such driving force transmitting means has a considerably longer service life than the consumption of the toner in the cartridge and the service life of the photosensitive drum (due to the fatigue thereof), since it has less wear and fatigue damage. However, in the conventional method wherein the driving force transmitting means is replaced and disposed together with the replacement and disposal of the cartridge, since the part (driving force transmitting means) which should not yet be disposed is replaced, there arose a problem that the cost of the cartridge and accordingly the running cost of the apparatus was increased, thus lessening the merit of the cartridge. This problem will be further noticable in the cartridges having a plurality of driving portions and used with the color image forming apparatus.